As is well known and accepted in the malt beverage brewing art, subjecting a hopped, especially alcoholic, malt brewery beverage, such as lager; ale, porter, stout and the like, (herein generically referred to as "beer"), to sunlight or artificial light, results in a significant deleterious effect on the sensory qualities of the beverage by generating the so-called "skunky" flavour which is sometimes also referred to as "sunstruck" or "light struck" flavour. It is believed that the sunstruck flavour is due to photochemical changes in the beverage which produce volatile sulphur-containing compounds. These sulphur compounds are thought to be formed at least in part by reaction of other sulphur-containing compounds with photochemically degraded hop components in the beverage and only very small amounts of these sulphur compounds are required to be present to impart the sunstruck flavour to the beverage and render it unacceptable. The photochemical reaction is assisted by the presence of riboflavin, one of several photo-initiators in the beverage, the riboflavin emanating mainly from the malt used in the production of beer and to a minor extent via the hops and, according to the common wisdom, the action of yeast during the fermentation (refer for example to "Kinetics of Riboflavin Production by Brewers Yeast" by Tamer et al. pages 754-756 Enzyme Microb Technology 1988 Vol 10 Dec.).
Attempts to prevent the beverages from becoming sunstruck involve enclosing the beer in cans or bottles made of protective, i.e. coloured glass, brown or amber being most efficient (refer for example to U.S. Pat. No. 2,452,968). These bottles reduce or eliminate the transmission to the beverage of light of wavelength shorter than about 560 nanometers, this light being most harmful since it assists the riboflavin in enhancing the production of the undesirable volatile sulphur compounds.
Another method developed to address the problem of "skunky" flavour production uses reduced isohumulones in place of hops or hop extracts, refer for example Verzele, M., et al., U. Inst. Brew. 73:255-257, 1967. Other methods involve adding light stabilizing materials to the beverage, see for example U.S. Pat. No. 4,389,421. However, in some jurisdictions, the use of such compounds have not been approved and further, many brewers are reluctant to use any additives and still use hops or hop extracts in attempting to achieve traditional beer flavour.
The problem of skunky flavour has been the subject of research for many years and such research continues, refer for example to the article "Sunstruck Flavour Formation in Beer" by Sakuma et al. ASBC Journal. This article also deals with the part believed played by riboflavin in the reaction of "skunky" flavour problem and suggests that removing riboflavin from the finished beer may solve the problem. However, an acceptable means for achieving that suggestion is clearly not readily apparent since the problem persists.
The most favourable context in which light is associated with malt beverages, is described in U.S. Pat. No. 3,787,587. That patent involves using an actinic light treatment on finished beer to accelerate the "aging" and "maturing" thereof. Moreover, sunlight or artificial light was used as early as 1865, refer U.S. Pat. No. 50,523, to treat the surface of boiling wort to decolorize it. The boiling wort, in keeping with typical brewery practices, would obviously have been hopped as well as being open to a highly oxygenated atmosphere during the boiling treatment. In both of these processes, it is almost a certainty that "skunky" beer flavour would ensue in the finished product. This is in keeping with what is well known in the art, about photochemical degradation in hopped malt beverages, and is in keeping with the general view that exposure of beer to light is anathema, and to be avoided at all costs in the interests of maintaining product shelf life.
It is also interesting to note that the literature teaches a process in which, sake, a unique Japanese fermented liquor or wine has been subjected to a light treatment--refer to Japanese published Patent Application 67667 entitled, "Process for Production of Sake Hardly Affected by Microbial Deterioration". This document teaches treating finished sake with light in the 200 to 700 micron range, the object being to have the light decompose riboflavin that is present in the finished sake, which riboflavin is an essential nutrient for lactic acid bacteria that is responsible for infectious spoilage in finished sake. As a consequence of the riboflavin destruction, the growth of the bacteria is inhibited and the sake preserved. Sake is, of course, a totally different product than beer, being an unhopped non-malt product, produced from rice using a starch-digestive enzyme amylase obtained from a mold and a special sake yeast to effect the fermentation. As stated, these differences materially distinguish sake from all malt brewery beverages. Note in particular, that riboflavin in sake, absent any lactic acid bacterial infection, does not pose off-flavour development problems such as those which are typically associated with hopped malt-containing products, such as beer.
The role of light in the production of hopped malt beverages, and especially in brewing, has received a great deal of attention, and it is generally accepted as axiomatic that in malt brewery beverages, especially those of an alcoholic nature, photochemical reactions produce accelerated aging, and in general will shorten a malt beverages shelf life and engender undesirable flavour development.